May 2004

With so many technological advancements in the fenestration industry during the past decade, it should be no surprise that adhesive technology is stronger than ever and offers window and door manufacturers benefits not possible a short time ago. Elastic bonding technology, the joining of substrates with an interfacial layer of permanently elastic adhesive, quickly is gaining the attention of leading manufacturers. From eliminating leaks and stress cracks to reducing stress concentrations in the bonded materials, elastic bonding can improve product life and enhance performance.

Elastic bonding technology long has been used in the construction, transportation, marine and appliance industries. In all, there are common requirements of the adhesive such as the ability to withstand constant resonant vibration and continuous hot and cold thermal cycling. These properties are also required in the fenestration industry and, as such, the benefits of elastic bonding technology are quickly gaining acceptance.

Elastic Bonding Benefits
When considering adhesives and sealants, window and door designers seemingly have concentrated their focus on tensile strength. While this is important, it is one of many factors that need to be considered when evaluating bonding agents.

Often manufacturers simply think of elastic bonding technology as using permanently elastic materials to bond similar or dissimilar substrates. While this is a key principle, elastic bonding is more than a product or material. It is a technology that improves not only long-term performance of the end product, but also the manufacturing process and often increases profit margins.

The fenestration industry benefits from the many advantages created by elastic bonding, such as uniform stress distribution, excellent joint movement compensation and gap filling, good resistance against impact stress, styling freedom and no deformation or damage to the substrates. This is particularly true when designing products for impact-, blast-resistance and high-performance applications.

With elastic bonding, window and door manufacturers have improved productivity, reduced costs and seen dramatic improvements in product performance and longevity as compared to rigid adhesives which require larger joint dimensions to compensate for lack of joint movement or risk joint failure. This is due in large part to fewer leaks, stress-induced failures and warranty claims than rigid bonding techniques which crack either the sealant or bonded substrate under fluctuating stresses and temperatures.
While lowering warranty claims excites most manufacturers, it is the improvements in productivity offered by elastic bonding that are often the biggest surprise. Elastic bonding technology allows manufacturers to tailor adhesive usage (many times using less adhesive than before) and design products and assembly processes for increased line flow. Elastic bonding
techniques most often allow for faster curing sealants or adhesives than rigid bonding techniques, which improves cycle time leading to less work in process and increased product shipments for manufacturers.

Keys to Elastic Bonding
Whether it’s a silicone, polyure-thane or a silane terminated polymer, there are a series of testing methods to consider before selecting a sealant or adhesive. Elastic bonding techniques require manufacturers to analyze and understand substrate and adhesive properties, manufacturing processes and application performance conditions. The results of this analysis, coupled with an optimized joint design, provide a solid base for an improved product.

Substrate Knowledge
The first step is to understand the surface of the various substrates to be bonded as well as their reactions to harsh environments and high and low temperature extremes.
Properties such as surface tension and coefficients of thermal expansion can be found in publications such as “Perry’s Chemical Engineers Handbook” or obtained from the material supplier. Surface tension can be used as a rough indication as to whether or not adhesion is possible, and to what degree. However, it should not be used as a substitute for full laboratory testing and validation of adhesion.

Initial Adhesion
and Durability
When designing a bonded system, it is necessary to understand the adhesive interfacial layer based on the effects of environmental cycling. Although a material may bond at room temperature, this is not an indication of adhesion when subjected to elevated or freezing temperatures or after severe environmental exposure. This information is critical when determining the life of a bond-line.

There are several methods for testing adhesion; however, the most widely used and accepted is the peel test which rates adhesion to a substrate after continuous exposure to a series of climates. Samples subjected to this test are exposed and evaluated after each of the following environments:
• 7 days; 23 degrees C; 50 percent relative humidity;
• 7 days; ambient water immersion;
• 7 days; 70 degrees C;
• 7 days; 70 degrees C; 100-percent relative humidity; and
24 hours; 30 degrees C.

A rating system for the peel test is based on the cohesive failure of the bond. One rating category is described as follows:
1. >95% cohesive;
2. >75% cohesive;
3. >25% cohesive;
4. <25% cohesive; and
5. Adhesive failure.

Following testing, an elastic bond will show cohesive failure. When the material is peeled from the substrate, a mass of material is left behind, which indicates the application was successful. Such failure indicates that the adhesion strength to the substrates actually exceeds the cohesive strength of the bonding or sealing material. Therefore, don’t rely on peel strength specifications alone. Review the cohesive failure percentages for accurate portrayals of adhesion performance.

Optimize Joint Design and
Optimize the Product
Elastic adhesives are susceptible to various exposures such as the environment, chemicals and load stresses. Similar to human skin, an adhesive joint should be protected from environmental exposure to reduce the effects of corrosion, ultraviolet radiation and environmental conditioning.

Stress analysis should be completed on the prototype product to determine the optimal minimum bond-line area, bead thickness and width. This analysis will improve the performance of the elastic joint in relation to the minimum strength requirement and the possibility of stress build-up in the bonded materials. This will also determine bond line material usage leading to faster cure rates than rigid bonds and elimination of excess material cost. When selecting an adhesive or sealant do not rely on tensile strength alone; rather achieve a balance of tensile strength, elongation, elasticity and recovery.

Application
Bead consistency is critical when using elastic bonding technology. Whenever possible, the recommendation is to use automation for consistent bead size, quality and placement. Application consistency will normally be reflected in less material usage, improved product performance and increased throughput in the assembly process.

Impacting
the U.S. Coasts
Fenestration manufacturers are now leveraging the benefits of elastic bonding for selection of new impact- and blast-resistant adhesives and sealants. The analysis process and associated products are also making their way into general fenestration applications including bedding and assembly processes, and ultimately field installation of the end products.

Further, window and door wall designs that incorporate elastic bonding technology considerations continue to pass the Miami Dade County PA 201 large missile and PA 203 cyclic wind pressure loading testing, usually on the first attempt. This generally is due to the adhesive or sealant being matched to the overall design and performance expectations of the system–through elastic bonding techniques.

While there are many criteria to use and review when choosing a sealant or adhesive system, keep the principles of elastic bonding in mind when bonding similar or dissimilar substrates. While the selection process will be more involved in the up-front system design, the end payoff includes better product quality and increased performance, adding significant savings to a manufacturer’s bottom line.